1. Field of the Invention
This invention relates to a device for splitting a light beam incident on an optical system by means of a diffraction lattice (or grating) structure.
2. Description of the Prior Art
The light metering apparatus in a camera splits a portion of an image forming light beam which has passed through a taking lens by a beam splitting device, and the thus split light is detected by a light detector. For the beam splitting device useful for such light metering apparatus, there has so far been used several devices. Such beam splitting device for use in a camera, etc. should desirably be as thin as possible. A thin beam splitting device employing a diffraction lattice structure is offered by Matsumoto in U.S. Pat. No. 4,103,153 and U.S. Pat. No. 4,172,646. This device splits the light beam incident on the device into a portion which can be used for light metering and the rest used in order to form a image by means of the diffraction lattice structure. On the other hand, a device formed by not using a diffraction lattice structure but using a plural number of small half mirrors obtained by dividing a half mirror to reduce the thickness is offered in U.S. Pat. No. 3,464,337. However, it is possible to reduce the thickness of the device further by using diffraction lattice structure.
FIG. 1 shows the cross sectional view of a single lens reflex camera employing the beam splitting device offered in U.S. Pat. No. 4,172,646.
In FIG. 1, 1 is a taking lens, 2 is a diaphragm, 3 is a quick return mirror, 4 is a film, 5 is a picture frame, 6 is a focusing plate, 7 is a condenser lens, 8 is a penta prism, 9 is an eye piece, 10 is a beam splitting device, and 11 is a light detector.
FIG. 2 shows the enlarged view of the beam splitting device shown in FIG. 1 and its surrounding section. The light splitting device consists of two parallel flat plates of glass 20 and 20' and a diffraction lattice structure 21 sandwiched between these glass plates. In other words the diffraction lattice structure 21 coated on both surfaces with adhesive is stuck to the two parallel flat plates of glass 20 and 20'. These glass plates 20 and 20' have almost the same refractive index as the diffraction lattice structure 21.
In FIG. 2 the light beam 12 coming from the taking lens 1 and incident on the beam splitting device is partially diffracted by the diffraction lattice structure 21 and the diffracted beam 14 reaches the glass end surface after being totally reflected by the boundary surface between the parallel flat plate glass 20 and 20' and air, and then led to the light detector 11. Moreover, the zero degree (zero order) transmitted light 13 is passed through a condenser lens 22, penta prism 8, and eye piece 9 and reaches the eye. Although it is possible to arrange a light metering device using such a beam splitting device as this at any position in a camera as long as the position is in the image forming light path, it is desirable, from the point of view of camera performance, to place it immediately before the condenser lens as shown in FIG. 1. However, effort has been made recently to push and compress the penta prism 8 toward the quick return mirror to reduce the size of the camera. For this purpose, the interval between the condenser lens and the focusing plate is desirably small and it is undesirable to allow the beam splitting device to occupy a wide space. This means that in the case where a beam splitting device offered in U.S. Pat. No. 4,172,646 is arranged between the condenser lens and focusing plate as shown in FIG. 2, and where the thickness of the beam splitting device is t.sub.1, the thickness of the condenser lens is t.sub.2, and the interval between the beam splitting device and the condenser lens is t.sub.3 , the thickness of the section where the condenser lens and the beam splitting device are coupled is expressed by T=t.sub.1 +t.sub.2 +t.sub.3 and the reduction in the thickness T leads to a more compact camera.